Metal vapor discharge lamp

ABSTRACT

A high-pressure mercury vapor discharge lamp of the additive type which includes in the discharge sustaining fill selected quantities of thallium iodide, dysprosium bromide and mercury and which, while evidencing qualities of luminous efficiency and color rendition comparable to similar lamps, exhibits superior lamp maintenance.

United States Patent [72] inventor Daniel A. Larson [5 References Cited Cedar Grove, N .J. UNITED STATES PATENTS 9F;- 32 3,259,777 7/1966 Fridrich 313/184 1e ar. 45 Patented Mar. 9, 1971 QTHER REFEIFENCES [73] Assignee Westinghouse Electric Corporation Factors Affectmg Metal Halide Lamp Parameters And Pittsburgh, Pa. Operating Characterics by Rokosz et a1, Illuminating Engineering, pp 626 630, November 1967. Class 313- 227 Primary Examiner-Raymond F. Hossfeld Attarneys-A. T. Stratton, B. R. Studebaker and W. D. Palmer [54] METAL VAPOR DISCHARGE LAMP 5 Claims 2 Drawing Flgs' ABSTRACT: A high-pressure mercury vapor discharge lamp [521 U.S. Cl 313/229, of the additive type which includes-in the discharge sustaining 313/184, 313/223, 313/228 fill selected quantities of thallium iodide, dysprosium bromide [51] Int. Cl H01j 61/18 and mercury and which, while evidencing qualities of lu- [50] Field of Search 313/184, minous efficiency and color rendition comparable to similar 225, 227, 228, 229 lamps, exhibits superior lamp maintenance.

PATENTEDHAR m 3,569,766

I87 20 3e 2 I4 FIG. I. 2o '2 I6 24 2 0 90 DYSPROSIUM BROMIDE LAMP 80\ 6Q- DYSPROSIUM- IODIDE LAMP 50- FIG. 2.

EFFICIENCY /w) I I l I I I l I l I I00 200 300 400 500 600 700 800 900 I000 HOURS WITNESSES INVENTOR S Q G DomelALorson gm d y g &.mm

ATTORNEY l METAL VAPOR DTSCHARGE LAMP BACKGROUND OF THE INVENTION The high-pressure mercury vapor discharge lamp has become increasingly familiar as the highly efficient bluish white light that brightens many of the highways and city streets across the nation. The mercury lamp has developed slowly but steadily into a highly efficient and reliable long-life lamp. One great disadvantage of the mercury lamp, however, is the characteristic blue-white light which it emits and its extremely poor color rendition. More recently the addition of metal halides to the mercury discharge, singly or in combination, to produce the spectral characteristics of the metals have permitted efiiciency increases of up to 100 percent to be obtained as well as radical improvement in color with increased lamp efficiencies of 50 percent and more. Many difiiculties have been encountered with respect to metal additives which from their spectral properties would appear to be desirable additives, either because they chemically attack the quartz envelope or have vapor pressures which are too low at reasonable envelope temperatures. At one time most commercially successful additive lamps included sodium generally in the form of the iodide in the discharge sustaining filling because of the significant increase in efficiency provided by that additive. A particularly successful lamp includes sodium, indium and thallium iodides as additives which produced a comparatively white appearance and an efficiency of 80 to 85 lumens per watt. More recently, a lamp which is the subject matter of copending application Ser. No. 599,133 filed Dec. 5, 1966 now U.S. Pat. No. 3,452,238 by Daniel A. Larson for a Metal Vapor Discharge Lamp and owned by the assignee of the present invention, was introduced. This lamp containing selected quantities of mercury, thallium iodide, mercuric iodide and dysprosium metal exhibited fine color, excellent color rendition, particularly in the red region of the spectrum, as well as high efiiciency, and made possible the elimination of sodium, along with its many deleterious effects, from a commercially acceptable mercury additive lamp.

Although the above described dysprosium iodide lamp provides an exceptional light source, it has been found to be subject to the same wall darkening of the arc tube, during life, that affects all iodine containing lamps. This wall darkening will cause the lamp to heat up and the efiiciency of the lamp to decrease during life. For example, the dysprosium iodide lamp although having an initial efficiency of about 90 lumens per watt will, after 1000 hours, have a reduction in efiiciency of about 28 percent. This failing is evidenced by all discharge lamps employing metal iodides in the discharge sustaining fill. Although the use of other metal halides have been suggested, none, to date, have been successfully employed in a mercury additive lamp.

SUMMARY OF THE INVENTION This invention relates to high pressure mercury vapor discharge lamps of the so-called additive type and more particularly to a discharge lamp having significantly improved maintenance without a reduction in color rendering properties and including dysprosium bromide as a principal discharge sustaining constituent.

An object of this invention is to provide a high-pressure mercury vapor discharge lamp of the additive type exhibiting significantly improved lamp maintenance, during the life of the lamp.

Another object of this invention is to provide a high-pressure mercury vapor discharge lamp having improved maintenance, high efficiency, good color rendition and which eliminates the deleterious effects of tungsten being deposited on the walls when tungsten electrodes are used.

The foregoing objects are accomplished in accordance with the present invention by providing, in a high-pressure mercury vapor discharge lamp, a discharge sustaining filling which consists essentially of a quantity of mercury which when fully vaporized during normal operation of the lamp will provide a predetermined mercury vapor pressure in the arc tube, plus selected quantities of thallium iodide, mercury and dysprosium bromide which is formed by initially charging the lamp with predetermined amounts of thallium iodide, mercuric bromide, dysprosium metal and mercury.

THE DRAWINGS The foregoing objects, and others, along with many of the attendant advantages of the present invention will become more readily apparent and better understood as the following detailed description is considered in connection with the accompanying drawings, in which:

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now in detail to the drawings, and more particularly to FIG. 1 there is shown the general arrangement of a high pressure mercury vapor discharge lamp within which the additive materials of the present invention are utilized. The

lamp, generally designated 10, includes a radiation transmitting sealed outer envelope 12 which is sealed to a standard mogul base 14. Mounted within the outer envelope 12 and spaced therefrom is'an inner envelope or arc tube 16. The are tube 16 is mounted within the outer envelope 12 by a conventional frame 18 and a pair of straps 20. Sealed within the inner envelope or are tube 16 disposed at opposite ends thereof are a pair of tungsten operating electrodes 22 and 24. The electrodes 22 and 24 are sealed through the opposite ends of the arc tube 16 by conventional ribbon seals 26. A starting electrode 28 is also sealed through the arc tube adjacent to electrode 24 by means of a ribbon seal 26.

The frame 18 is carried by one of a pair of conventional lead-in conductors 30 which extend through a conventional reentrant stern press 32 connected to mogul base 14 which in turn is connected to a conventional power source 34 in the well known manner.

The electrodes 24, 22 and 28 are electrically connected to one or the other of the lead-in conductors 30. A starting resistor 36 is connected between one of the lead-in conductors 30 and the starting electrode 28 through the frame 18.

The lamp configuration is essentially conventional and a more detailed description of its operation may be found in U.S. Pat. No. 2,748,303 dated May 29, 1956 to Thorington.

in order to maintain sufticiently high temperatures in the normally coldest areas of the arc tube, (i.e. that portion behind each of the electrodes) a heat retaining coating of, for example, a zirconium oxide-silicon dioxide coating, which is applied as a suspension with nitrocellulose, may be adhered to the portion of each end of the arc tube surrounding an elec trode. This end coating will aid in maintaining a cold spot temperature within the arc tube of not less than 650 C.

The predecessor to the lamp of this invention employed thallium iodide, mercuric iodide, dysprosium metal and mercury in the discharge sustaining fill during initial manufacture of the lamp. Upon operation the mercuric iodide combined with the dysprosium to form mercury and dysprosium iodide which provided the substantial red component in the discharge. Although this lamp provided the best color rendition available from a mercury additive lamp, it was found that during life the wall of the arc tube would darken due to the tungsten deposits from the electrode and cause heating up and failure of the lamp. Quite unexpectedly it has been found that by substituting mercuric bromide for the mercuric iodide in the prior art lamp that the wall darkening is substantially reduced and the lamp maintenance is substantially improved. This phenomenon is believed to occur due to the introduction of a tungsten-halogen cycle by the bromine which cleans up the tungsten. iodine will evidently not precipitate operation of a tungsten-halogen cycle clean up.

The lamp of this invention has a discharge sustaining fill during operation of the lamp which includes predetermined quantities of thallium iodide, dysprosium bromide and mercury. The initial charge 38 for the lamp during manufacture is with predetermined quantities of thallium iodide, mercuric bromide, dysprosium metal and mercury. ideally the lamp would be initially charged with dysprosium bromide, but for reasons of instability in air, dysprosium bromide is difficult to handle in the manufacture of the lamp.

As a specific example, the arc tube 16 has an internal volume of 12 cubic centimeters, the space between the tips of electrodes 22 and 24 is about 51 millimeters, with the arc tube wall being approximately 1 millimeter thick. Mercury vapor lamps of this type are generally operated at 400 watts and contain a starting gas of for example,argon, at a pressure of about 20 to 25 torrs. Thespecific embodiment would further include as a part of the discharge sustaining fill a quantity of mercury of about 32 to 40 milligrams which when operated at the rated wattage with a cold spot temperature of between 650 and 700 C would completely vaporize and reach a pressure of several atmospheres. When the mercury is fully vaporized a voltage drop across the lamp of approximately 135 volts will be established.

The constituents in addition to mercury are from about 1.0 to 10.0 milligrams of thallous iodide,- from about 5.0 to 15.0 milligrams of mercuric bromide, and from about 1.0 to 5.0 milligrams of dysprosium metal. More importantly than the specific amounts of mercuric bromide and dysprosium metal is the fact that these two constituents should be present in the initial charge of the lamp in nearly stoichiometric amounts with a slight excess of dysprosium metal due to the impurity of commercially available dysprosium metal and the formation of dysprosium oxide in handling.

The preferred embodiment of the lamp will include in the above described arc tube an initial charge of about milligrams of T1], about milligrams of HgBrabout 3.5 milligrams of Dy and about 36 milligrams of Hg. After a short period of initial operation during the manufacture of the lamp which is known in the art as seasoning, these ingredients will convert to form about 8 milligrams of DyBr about 42 milligrams of Hg and 5 milligrams of T1].

The reason for initially charging the lamp with mercuric bromide and dysprosium metal in nearly stoichiometric amounts is because the presence of too little dysprosium affects the color rendition, particularly the red contribution, while too much dysprosium will cause brown spots to form on the arc tube and affect lamp efficiency.

It will be apparent that for arc tubes of other configurations the necessary constituents required to produce the lamp of the present invention are from about 6.4 to 8.0 milligrams of mercury per centimeter of arc length, from about 0.2 to 2.0 milligrams of thallium iodide per centimeter of arc length, from about 0.2 to 1.0 milligrams of dysprosium metal per centimeter of arc length, and from about 1.0 to 3.0 milligrams of mercuric bromide per centimeter of arc length.

To construct the preferred embodiment of the lamp in arc tubes of different sizes the preferred discharge sustaining fill would include about 1.0 milligram per centimeter of arc length of thallium iodide, about 2.0 milligrams of mercuric bromide per centimeter of arc length, about 0.7 milligrams of dysprosium metal per centimeter of arc length and about 7 milligrams of mercury per centimeter of arc length.

As will be seen from FIG. 2, both the bromide lamp and the iodide lamp have substantially the same initial efficiency in terms of lumens per watt. The initial efficiency of the bromide lamp was about 91 lumens per watt while the iodide lamp was about 90 lumens per watt. After the first 200 hours of operation there is a substantial drop in the efficiency of the iodide lamp as the tungsten from the tungsten electrode is deposited on the walls of the arc tube, and the efficiency of the iodide lamp has dropped about 14 lumens per watt while the drop in the bromide lamp is only about 6 lumens per watt. The remainder of FIG. 2 indicates that this approximate lO-percent in efficiency is maintained throughout the life of the lamp and is still a difference of about 9 lumens per watt at 1000 hours. This unexpected improvement in lamp maintenance of about 10 percent or 9 lumens per watt is substantial in terms of lamp industry performance.

As will be seen from the foregoing, the lamp of this invention essentially eliminates the deposition of tungsten on the lamp walls through the bromine combining with the tungsten in what is known as a tungsten-halogen cycle reaction. Such does not occur when Dysprosium Iodide is present. By substituting mercuric bromide for the mercuric iodide and thus essentially producing a dysprosium bromide discharge there is maintained the spectacular color rendition of the iodine lamp but the undesirable drop in efficiency is eliminated to a great extent.

Since numerous changes may be made in the above described lamp and different embodiments of the invention may be made without departing from the spirit thereof, it is intended that all matter contained in the foregoing description or shown in the accompanying drawings, shall be interpreted as illustrative and not in a limiting sense.

lclaim:

1. An improved discharge lamp comprising:

a radiation transmitting elongated arc tube enclosing a predetermined volume;

arc supporting tungsten electrodes disposed within said are tube proximate the ends thereof and defining a predetermined arc length;

lead-in conductors connected to said electrodes and sealed through said are tube; and

a discharge sustaining filling within said arc tube which initially consists essentially of a predetermined quantity of mercury which when fully vaporized during normal operation of the lamp will provide a predetermined mercury vapor pressure in said arc tube plus predetermined quantities of thallium iodide, mercuric bromide, and dysprosium metal whereby a tungsten-halogen cycle is created substantially improving lamp efficiency during life.

2. An improved discharge lamp according to claim 1 wherein said mercury is initially present in an amount of from about 6.4 to 8.0 milligrams per centimeter of arc length, said thallium iodide is initially present in an amount of from about 0.2 .to 2.0 milligrams per centimeter of arc length, said mercuric bromide is initially present in an amount of from about 1.0 to 3.0 milligrams per centimeter of arc length, and said dysprosium metal is initially present in an amount of from about 0.2 to 1.0 milligrams per centimeter of arc length.

3. An improved discharge lamp according to claim 2 wherein upon initial operation of said lamp said dysprosium is present in dysprosium bromide, and the relative gram atom proportions of total dysprosium to total bromide is generally such as to form DyBr 4. An improved discharge lamp comprising:

a radiation transmitting elongated arc tube enclosing a predetermined volume;

arc supporting tungsten electrodes disposed within said are tube proximate the ends thereof and defining a predetermined arc length;

lead-in conductors connected to said electrodes and sealed through said arc tube; and

a discharge sustaining filling within said arc tube which initially consists essentially of a quantity of mercury which when fully vaporized during normal operation of the lamp will provide a predetermined mercury vapor pressure in said are tube plus selected quantities of thallium iodide, mercuric bromide and dysprosium metal, said thallium iodide being initially present in an amount of from about 0.2 to 2.0 milligrams per centimeter of arc length, said mercury being initially present in an amount of from about 6.4 to 8.0 milligrams per centimeter of arc length, said mercuric bromide being initially present in an amount of from about l.0 to 3.0 milligrams per centimeter of arc length, and said dysprosium metal being initially per centimeter of arc length, said dysprosium metal is initially present in an amount of about 0.7 milligrams per centimeter of arc length and mercuric bromide is initially present in an amount of about 2.0 milligrams per centimeter of arc length. 

2. An improved discharge lamp according to claim 1 wherein said mercury is initially present in an amount of from about 6.4 to 8.0 milligrams per centimeter of arc length, said thallium iodide is initially present in an amount of from about 0.2 to 2.0 milligrams per centimeter of arc length, said mercuric bromide is initially present in an amount of from about 1.0 to 3.0 milligrams per centimeter of arc length, and said dysprosium metal is initially present in an amount of from about 0.2 to 1.0 milligrams per centimeter of arc length.
 3. An improved discharge lamp according to claim 2 wherein upon initial operation of said lamp said dysprosium is present in dysprosium bromide, and the relative gram atom proportions of total dysprosium to total bromide is generally such as to form DyBr3.
 4. An improved discharge lamp comprising: a radiation transmitting elongated arc tube enclosing a predetermined volume; arc supporting tungsten electrodes disposed within said arc tube proximate the ends thereof and defining a predetermined arc length; lead-in conductors connected to said electrodes and sealed through said arc tube; and a discharge sustaining filling within said arc tube which initially consists essentially of a quantity of mercury which when fully vaporized during normal operation of the lamp will provide a predetermined mercury vapor pressure in said arc tube plus selected quantities of thallium iodide, mercuric bromide and dysprosium metal, said thallium iodide being initially present in an amount of from about 0.2 to 2.0 milligrams per centimeter of arc length, said mercury being initially present in an amount of from about 6.4 to 8.0 milligrams per centimeter of arc length, said mercuric bromide being initially present in an amount of from about 1.0 to 3.0 milligrams per centimeter of arc length, and said dysprosium metal being initially present in an amount of from about 0.2 to 1.0 milligrams per centimeter of arc length.
 5. An improved discharge lamp according to claim 4 wherein said mercury is initially present in an amount of about 7.0 milligrams per centimeter of arc length, said thallium iodide is initially present in an amount of about 1.0 milligram per centimeter of arc length, said dysprosium metal is initially present in an amount of about 0.7 milligrams per centimeter of arc length and mercuric bromide is initially present in an amount of about 2.0 milligrams per centimeter of arc length. 